1. Field of the Invention
The prevent invention generally relates to a method of fabricating an integrated circuit (IC) and more particularly to a method of fabricating a high density flat cell mask read only memory (ROM) using the trench technique to resolve the problem of cell punch-through.
2. Description of Related Art
Memory devices are widely used in different kind of information hardware. Since hardware has become lighter, thinner, shorter and smaller, high density and small and fast memory is required. Of the different kinds of memory, a ROM is a permanent memory, which is used in digital equipment, such as mini computer systems, micro computer systems, to store permanent programs of these systems. The production process of a ROM is very complicated including many steps. Each step spends time in material processing and factor control. Because most ROMs have the same basic structure except for information stored in programming, in most IC factories, a half product (partially completed product) of a mask ROM is produced without performing programming and is stocked. After receiving an order for a special program from customer, a mask is produced to perform the programming and to finish production, so that the lead time can be reduced to provide better service for customers. Therefore, a post programmed mask ROM is commonly used in the art.
Currently, the mask read only memory with buried bit lines is one of the most popular mask ROMs due to its small dimensions and high density. FIG. 1 is a top-view diagram illustrating part of traditional flat mask ROM wherein source-drain electrode areas 10 are buried bit lines, and polysilicon gates 11 are word lines. The area marked with numeral 12 is a memory cell. FIG. 2 is a schematic cross-sectional diagram illustrating the cross-section along the direction of line II--II in FIG. 1, wherein numeral 20 indicates a silicon substrate and the numeral 21 indicates a gate oxide layer. The buried bit lines 10 are formed by implantation. After the bit lines 10 are formed, there are may processes to be performed to complete the whole integrated circuit. That is, the wafers will undergo several high temperature processes. Each high temperature process could cause the diffusion of impurities in area such as the buried bit lines 10, so that the space s between two adjacent buried bit lines 10 will become narrow. For example, in FIG. 2, the space s between two adjacent source-drain electrode areas 10 becomes narrow which induces cell punch-through.
As the need of high density of memory is stronger than ever, the most efficient method to increase the density of a high density flat memory with buried bit lines is to diminish the dimensions of each memory cell. That is, to diminish the space between the source electrode and the drain electrode of a memory cell. In this way, the punch-through problem between two impurity areas will become more and more serious.